## Problem
We don't have any logging for Sentry initialization. This makes it hard
to verify that it has been configured correctly.
## Summary of changes
Log some basic info when Sentry has been initialized, but omit the
public key (which allows submitting events). Also log when `SENTRY_DSN`
isn't specified at all, and when it fails to initialize (which is
supposed to panic, but we may as well).